Encapsulation for an electrical component and method for producing the same

ABSTRACT

Sensitive component structures ( 2 ) can be encapsulated by enclosing them with a frame structure ( 6 ) consisting of a light-sensitive reaction resin and covering the latter with another, structured layer of reaction resin after applying an auxiliary film ( 7 ). Top structures ( 10 ) which fit over the frame structures ( 6 ) can be produced e.g., by structured imprinting or photostructuring. The residual parts of the exposed auxiliary film are removed by dissolving or etching.

[0001] In the manufacture of electrical and electronic components fromwafers, it is often necessary to provide sensitive component structureswith a cover while they are still on the wafer, i.e., prior to theirbeing separated into individual components. This cover can, for onething, serve as protection against external influences further along inthe manufacturing process or can become the foundation for certainpackaging technologies. A cover of this type can form a mechanicalprotection against media introduced further along in the process or as aprotection against contamination, in particular against electricallyconductive particles, which are particularly disruptive in miniaturizedcomponents. The packaging can be completed based on this cover, forexample in a resin mold, compressing it in plastic, installing it inanother housing, etc.

[0002] Surface acoustic wave devices have especially sensitive componentstructures that can include inter-digital transducers and reflectors inthe form of finely structured conduction paths. These sensitivestructures are not, in general, permitted to come into contact with thecovers noted above, since this contact can cause properties of thesurface acoustic wave devices to change in an unreliable andnon-reproducible manner. To cover such structures requires anencapsulation that forms a cavity over the structures.

[0003] From WO 95/30 276, an encapsulation for electrical and electroniccomponents is known that comprises a frame and a support structure thatencloses the component structures and a cover layer that sits on top ofthem that can form a hermetically sealed cap, together with the frame,that encloses the sensitive structures. It is preferred that both theframe structure as well as the cover layer be made of a photoresistmaterial, and particularly of a photoresist foil. These are laminatedover the entire surface, exposed via a mask, and developed. These stepsare separately carried out for the frame structure and the cover layer.Overall, the process is relatively intensive and requires a multitude ofprocess steps.

[0004] The object of this invention is thus to provide a process toencapsulate sensitive component structures that is simpler to carry out,yet leads to a secure encapsulation of even sensitive componentstructures.

[0005] This object is met according to the invention by a processdescribed in claim 1. Advantageous embodiments of the invention as wellas an encapsulation thus produced can be found in the other claims.

[0006] Like the known encapsulation just described, the processaccording to the invention is based on a two-part encapsulation system,namely a frame structure that encloses the component structures and aceiling structure that sits on top of it. According to the invention,however, the difference from the known process is that the reactiveresin is applied in fluid form both for the frame structures as well asfor the ceiling structures, said resin being cured afterward. After theframe structures are established by the exposure and development of afirst reactive resin layer, an auxiliary foil is stretched over theentire surface, if necessary over the entire wafer, and not just overthe individual component. This foil covers all frame structures producedon the substrate, and adheres to them. The second reactive resin layeris then applied on top of the auxiliary foil in a structured manner. Inthe last step, the areas of the auxiliary foil outside the framestructures and between the ceiling structures are removed, preferably bydissolving them using a solvent or by treating them with a plasma.

[0007] In a preferred embodiment of the invention, a UV-cured reactiveresin is used for the second reactive resin layer that is againstructurally exposed, developed and cured after being applied to theentire surface.

[0008] A special advantage is that it is very simple and controllable tospin coat the fluid reactive resin, and it is easier to execute than thelamination of resist foils. It is also more cost effective overall. Thesecond reactive resin layer provided for the ceiling structure can thenbe applied in any desired thickness to ensure sufficient mechanicalstability for the encapsulation. The application of further layers overthis ceiling structure is then not required.

[0009] In the process, the image exposure of the first and secondreactive resin layer can each be done using a laser, in particular usinga UV (ultraviolet) laser. This is advantageous, since the process can beeasily adjusted according to varying component structures, eliminatingthe need to first produce expensive photo masks for the exposureprocess.

[0010] Advantageously, parallel to the manufacture of the framestructures in the first reactive resin layer, other component structurescan be produced at the same time. Examples of such structures includedamping structures in surface acoustic wave devices. In this case, it isadvantageous if the material of the reactive resin layer is matchedacoustically, i.e., having a suitable hardness and a suitable modulus ofelasticity.

[0011] It is preferred to develop the reactive resin layers using afluid developer. Depending on the resin system used, the fluid developermay be an organic solvent or an aqueous-alkaline solvent.

[0012] In another embodiment of the invention, the second structuredreactive resin layer is produced by structured printing onto theauxiliary foil in the area over the frame structures and then curing.Suitable structured printing processes include screen printing andstencil printing. Thanks to the auxiliary foil, the printing is notcritical even when the structural dimensions of the component structuresare small, since it is only necessary to cover the frame structures withthe second reactive resin layer. The printing precision of the printprocess noted above is sufficient for this purpose even if the printingprocess is not suited for producing the first reactive resin layer.

[0013] This auxiliary foil can be a thin plastic foil, and in particularit can be a thin thermoplastic foil. This is advantageous since itallows the auxiliary foil to be easily stretched, producing an evencovering over the frame structures. Furthermore, the plastic foil canconnect to the frame structures easily, e.g., by adhesion, melting orfusing.

[0014] In the process, the material for the plastic foil can be selectedsuch that it can be easily removed. The material may be soluble in asolvent used to remove the plastic foil in the areas between the framestructures that are not for the purposes of encapsulation.

[0015] An appropriately thin plastic foil can also be easily removed inplasma, in particular in plasma containing oxygen. In the process, thethickness of the second reactive resin layer is selected to providesufficient excess to provide for the dissolution that acts on thissecond reactive resin layer as well during this removal process.Suitable materials for the plastic foil include, e.g., polyamide, PET,or polycarbonate foils. In the process, the thickness of the plasticfoil is selected so that on one hand it is as thin as possible, and onthe other hand it is sufficiently sturdy so as to support the secondreactive resin layer without sagging too much. Usually, foil thicknessesnear 1 μm are sufficient. However, thicker foils of up to approximately20 μm can also be used, as can thinner foils. For example, the foilthickness can be from 0.5 to 5 μm.

[0016] The following explains the invention in more detail with the helpof representative examples and with the help of nine figures.

[0017]FIGS. 1 through 8 show various process steps to produce theencapsulation of the present invention, shown by schematic crosssections through a component to be encapsulated.

[0018]FIG. 9 shows a plan view of the component during a stage in theprocess.

[0019] This embodiment is directed to the encapsulation of active, andthus sensitive, component structures of a surface acoustic wave device.The explanatory figures are only schematic representations and thus arenot to scale.

[0020]FIG. 1 shows a piezoelectric substrate 1, such as a wafer made oflithium tantalite or lithium niobate. Various component structures areapplied to the surface of the substrate 1. Examples of such structuresinclude electrically conducting structures such as an aluminum (e.g.,metal) layer. In FIG. 1, only sections of this metal layer are shown,namely the sensitive component structures 2, which can belong todifferent components although they are located on a single wafer. Alight-sensitive reactive resin layer 3 is spun onto the entire surfaceof the substrate 1 over the component structures. The resin layer 3 mayhave a thickness of approximately 50 μm. The thickness of the reactiveresin layer 3 is selected so that it exceeds the thickness of thecomponent structures 2 to a sufficient extent that a height differencebetween the top edge of the component structures 2 and the reactiveresin layer 3 ensures a safe distance for the cover of the componentstructures 2.

[0021] It is preferred to apply a cationically initiated UV-cured,solvent-free epoxy resin as the reactive resin, cured by UV radiation.This reactive resin contains another photo initiator in addition to thecationically cured epoxy or a photo initiator system that matches theexposure source. These types of epoxy resins are described in DE-A 44 43946, for example, which is incorporated herein by reference in itsentirety. To support curing, the reactive resin can contain additionaladditives, preferably of a basic nature and selected from the group ofsalt hydroxides or organic amines.

[0022] Referring to FIG. 2, through scanned exposure 4 by a UV laser,certain areas 5 in the first reactive resin layer 3 are exposed and atleast partially cured. The exposure causes a solubility gradient tooccur between the exposed areas 5 and the unexposed areas of the firstreactive resin layer 3, allowing the development to be carried out usinga developer solution.

[0023]FIG. 3 shows the component after development. The exposed areas 6of the original first reactive resin layer 3 that remain afterdevelopment form closed frame structures 6 that enclose the sensitivecomponent structures 2. The frame structures 6, if they are onlypartially cured, can be completely cured in this process step byincreasing the temperature for a short period. The complete cure canalso occur prior to development.

[0024] A thin plastic foil 7 is thus stretched over the frame structures6 such that the thin plastic foil sits on top of the frame structures 6.It is preferable to stretch the thin plastic foil 7, which is used as anauxiliary foil, over the entire substrate 1.

[0025]FIG. 4 shows the arrangement resulting after foregoing processstep. By heating and layering the foil with adhesive, through laser orfrictional fusing or a similar means, the foil 7 is made to stick to theframe structures 6.

[0026] A fluid layer 8 of an equally light-sensitive reactive resin isthen spun onto the entire surface of the auxiliary foil 7. Preferably,the same resin as was used for the first reactive resin layer is usedhere. The thickness of this second reactive resin layer 8 is selectedsuch that it is considerably more than the thickness of the plastic foil7. FIG. 5 shows the arrangement resulting after this process step.

[0027] A structured exposure is then performed on the second reactiveresin layer 8 to produce cured areas 9. The exposure is carried out inthe same manner as the exposure of the first reactive resin layer 3,e.g., by scanned UV laser radiation. The cured areas 9 are arranged suchthat they form ceiling structures that fit over the frame structures 6,sealing them shut at their outer perimeter (see FIG. 6).

[0028] Referring to FIG. 7, if necessary, another temperature varyingstep is now carried out to complete the curing in areas 9. Then, theunexposed areas of the second reactive resin layer 8 are removed using afluid developer, leaving the ceiling structures 10 behind. Depending onthe resin system used, the developer can be an organic solvent or anaqueous alkaline solvent.

[0029] In the next step, the areas of the plastic foil 7 not covered bythe ceiling structures 10 are removed, for example, through short-termincineration treatment in suitable plasma, which may contain oxygen. Inthis etching step, the open, uncovered areas of the plastic foil 7 arecompletely removed. At the same time, some of the ceiling structure 10layer is removed. FIG. 8 shows the resulting completely encapsulatedcomponent structures 2. The encapsulation includes the frame structure 6and the ceiling structure 10 with auxiliary foil 11 residue in between.The component structures 2 are thus hermetically sealed and protectedagainst further aggressive process steps. It is also possible toseparate the components in this stage by subdividing the substrate 6between the component structures, i.e., the encapsulated components. Theseparation can be performed using a saw, for example.

[0030]FIG. 9 shows the arrangement of the frame structures 6 around thesensitive component structures 2. They are shown only schematically. Thesensitive component structures can be connected to electrical connectingsurfaces 12 on the substrate through conduction paths. The framestructure 6 is then arranged so as to enclose the sensitive componentstructures 2, but the connection surfaces 12 and, if necessary, part ofthe electrical leads to it sit outside the frame structure. In theprocess, part of the electrical lead is covered by the frame structure.After applying the ceiling structures 11, 10, the outer edges of whichcoincide with the perimeter of the frame structures 6, the electricalconnection surfaces 12 remain accessible and can be contactedexternally. For example, this can be done using flip chip bonding, inwhich the connection surfaces 12 are connected to a base plate usingso-called bumps. It is, however, also possible to connect to thecomponent via the connection surfaces 12 using bonding wires.

[0031] Further processing is preferred to be through flip chip bonding,with the sensitive component structures sitting on the surface of thesubstrate that faces the base plate, and are thus additionallyprotected. By sealing the gaps between the substrate 1 and the baseplate as well (not shown in the figure), the component can be furthersealed.

[0032] Although the invention was described by means of one embodiment,it is not limited to that embodiment. To the contrary, it is possible toencapsulate other components with the aid of the invention using othersubstrate materials, other reactive resins or another type of exposureas well. The type of component to be encapsulated, i.e., the componentstructures to be encapsulated, also determine the geometric dimension ofthe encapsulation, which can be widely varied.

1. A process to selectively encapsulate sensitive component structures(2) of electric components with the following steps: spin-coating afluid, light-sensitive first reactive resin layer (3) onto the entiresurface of the component substrate (1) containing the sensitivecomponent structures, image exposure and development of the firstreactive resin layer, whereby a frame structure (6) remains thatencloses the component structures (2), stretching of an auxiliary foil(7) over all frame structures (6) on the substrate (1), producing asecond structured reactive resin layer (10) on the surface of theauxiliary foil above the frame structure (6) so that ceiling structures(10) remain that make a seal with the frames (6) and constitute a cavitytogether with them, removal of the auxiliary foil (7) in the free areasbetween the ceiling structures (10).
 2. A process according to claim 1,in which the production of the second structured reactive resin layer(8) includes the following steps: spin-coating of a fluid,light-sensitive second reactive resin layer (8) onto the entire surfaceof the auxiliary foil (7), image exposure and development of the secondreactive resin layer, wherein ceiling structures (10) remain that make aseal with the frames (6) and form a cavity together with them.
 3. Aprocess according to claim 2, in which the image exposure of the firstand/or the second reactive resin layer (3, 8) is done by a laser (4)scan.
 4. A process according to claim 1 or 2 in which the development ofthe exposed first and/or second reactive resin layer (3, 8) is doneusing a fluid developer.
 5. A process according to claim 1, in which thesecond structured reactive resin layer (8) is applied in a structuredmanner to the auxiliary foil (7) using a printing process.
 6. A processaccording to claim 5, in which the ceiling structures (10) are appliedand then cured using a screen or stencil printing process.
 7. A processaccording to one of claims 1-6, in which a thin plastic foil is used asthe auxiliary foil (7).
 8. A process according one of claims 1-7, inwhich the auxiliary foil (7) is adhered or fused to the frame structures(6).
 9. A process according to one of claims 1-8, in which the removalof the remaining auxiliary foil (7) between the ceiling structures isdone using an incineration treatment by a plasma.
 10. A processaccording to claim 1-8, in which the removal of the remaining auxiliaryfoil (7) between the ceiling structures is done by treating it with asolvent.
 11. A process according to one of claims 1-10, in which inaddition to the frame structures (6), damping structures are formedduring the image exposure and development of the first reactive resinlayer (3).
 12. A process according to one of claims 1-11, in which theauxiliary foil (7) is selected from polyamide, PET or polycarbonatefoils and has a thickness of 0.5-5 μm.
 13. A process according to one ofclaims 1-12, in which the first and second reactive resin layer (3, 8)are a UV-cured epoxy resin.
 14. An encapsulation for an electricalcomponent containing sensitive component structures (2) on a substrate(1), with a frame structure (6) made of a cured reactive resin on thesubstrate that encloses the sensitive component structures, and with acap that covers the frame structure (6) and that constitutes a sealedcavity together with it, consisting of a plastic foil (11) and a curedreactive resin layer (10) on top of it.
 15. An encapsulation accordingto claim 11, in which the plastic foil (7) is a thermoplastic,alternatively made of polyamide, PET or polycarbonate.